Their results could help scientists better predict the storms, which can endanger astronauts, as well as disrupt power, satellites and communications.

The Earth's magentic field, or magnetosphere, takes the brunt of solar storms caused by coronal mass ejections, the Sun's regular emission of charged gas called plasma. But some of the charged particles slip through cracks in the magnetosphere and cause problems for Earth.

This study is part of an ongoing effort by scientists to try and understand how such cracks are formed.

"Before, scientists thought the cracks would open and close on a timescale of just a few minutes," said Frey. "We have seen that these cracks can stay open continuously for many hours, these observations solve an old controversy about the nature of the crack forming process."

"Now that we know these cracks do not just open and close sporadically ... but can stay open for a long and extended time of several hours ... we can go on and incorporate this knowledge into our next step of modeling space weather impacts."

The new observations were possible using NASA's Imager for Magnetopause to Aurora Global Exploration (IMAGE) orbiting satellite, which watched the Earth's magnetosphere. A group of four European Space Agency satellites called Cluster were also involved. They detected the charged solar particles streaming through the crack in the magnetic field.

Frey's team studied a collision between the solar wind and Earth's magnetic field that occurred in 2002. During the study, IMAGE revealed an area of heavy particles called ions almost the size of California hitting the ionosphere above the Arctic region: "This aurora, energetic enough to power 75,000 homes, was different from the visible aurora known as the Northern and Southern lights," NASA said.

"While the aurora was being recorded by IMAGE, the 4-satellite Cluster constellation flew far above IMAGE, directly through the crack, and detected solar wind ions streaming through," NASA said. "Normally, these solar wind ions would be deflected by Earth's shield, so Cluster's observation showed a crack was present."

The researchers estimated the the crack was twice the size of the Earth at a point 60,000 km above the planet's surface. Such cracks evidently open all the time, said Professor Nancy Crooker of Boston University.

"It is critical for us to understand where, when, and for how long the magnetic shield is breached and how this energy gets in," space weather expert Dr Janet Kozyra of the University of Michigan commented. This information "will help us to make more accurate space weather predictions".

Several monster solar storms hit the Earth last month but caused little disruption. When officials know they are coming they can move satellites and prepare power grids to minimise the effects.